In some precision apparatuses such as precision machining equipment and precision measuring instruments, a material called Super Invar (32% Ni-5% Co—Fe), for example, which has an extremely small coefficient of thermal expansion, is sometimes used in a component that requires temperature-stable performance in order to reduce deformation and errors due to changes in ambient temperature.
However, when exposed to extremely low temperatures, a Super Invar material undergoes martensitic transformation, which changes the coefficient of thermal expansion of the Super Invar material to impair its original performance. Therefore, care needs to be taken to keep the part made of the Super Invar material from being exposed to extremely low temperatures lower than or equal to the martensitic transformation temperature.
To address the problem of martensitic transformation of such Ni—Co—Fe low thermal expansion materials, Japanese Patent Application Laid-Open No. 2001-11580 (hereinafter referred to as Patent literature 1) proposes an adjustment of components of an alloy to improve the stability of the alloy at low temperatures.
On the other hand, in the event that martensitic transformation occurs in a Super Invar material or other low thermal expansion material that undergoes martensitic transformation used in a precision apparatus, performance degradation or functional failure can occur.
If performance degradation or functional failure occurs, a temperature history, for example, can be used to determine whether or not the degradation or failure has been caused by martensitic transformation. That is, if means for obtaining the temperature history is provided in the precision apparatus, one can determine from the temperature history whether or not martensitic transformation has occurred due to exposure to extremely low temperatures.
Japanese Patent Application Laid-Open No. 2001-124715 describes a transformation observation apparatus including temperature detecting means and high-frequency current detecting means as means for determining whether transformation of metal has occurred or not.
As has been described above, provision of transformation observation equipment or temperature history obtaining means in a precision apparatus allows to readily determine whether performance degradation or functional failure in the apparatus has been caused by martensitic transformation of a low thermal expansion material used in the apparatus.
However, the provision of such transformation observation equipment or temperature history obtaining means adds cost to the precision apparatus, thereby making the precision apparatus expensive. Therefore, such transformation observation equipment or temperature history obtaining means have not commonly been adopted. In addition, transformation cannot be observed and a temperature history cannot be maintained while power is off such as during storage or transportation of the apparatus.
Therefore, it is a common practice that a precision apparatus is disassembled to take out a part made of a low thermal expansion material that might have undergone martensitic transformation and the surface (structure) of the part is observed or the dimensions or the coefficient of thermal expansion of the parts are precisely measured in order to determine whether or not performance degradation or functional failure has been caused by martensitic transformation. Such operation is laborious and time consuming.